Reflective spatial light modulator

ABSTRACT

A micro-mirror array that is useful, for example, in a reflective spatial light modulator. In one embodiment, the micro mirror array includes spacer support walls, a vertical torsion hinge, and a mirror plate, all being fabricated from a single wafer. The micro-mirror array has a large fill ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application is a continuation of and claimsthe priority benefit of U.S. patent application Ser. No. 10/378,056, for“Reflective Spatial Light Modulator,” filed Feb. 27, 2003, which patentapplication claims priority from provisional U.S. patent applicationSer. No. 60/390,389, for “Reflective Spatial Light Modulator,” filedJun. 19, 2002, the disclosures of which are incorporated by reference.

BACKGROUND

[0002] 1. Field of the Invention

[0003] This invention relates to spatial light modulators (SLMs), andmore particularly to a micro-mirror array with electronicallyaddressable control circuitry for display applications.

[0004] 2. Background of the Invention

[0005] Spatial light modulators (SLMs) have numerous applications in theareas of optical information processing, projection displays, video andgraphics monitors, televisions, and electrophotographic printing.Reflective SLMs are devices that modulate incident light in a spatialpattern to reflect an image corresponding to an electrical or opticalinput. The incident light may be modulated in phase, intensity,polarization, or deflection direction. A reflective SLM is typicallycomprised of an area or two-dimensional array of addressable pictureelements (pixels) capable of reflecting incident lights. Source pixeldata is first processed by an associated control circuit, then loadedinto the pixel array, one frame at a time.

[0006] Prior art SLMs have various drawbacks. These drawbacks include: alower than optimal optically active area (measured as what fraction ofthe device's surface area that is reflective, also called the “fillratio”) that reduces optical efficiency, rough reflective surfaces thatreduce the reflectivity of the mirrors, diffraction that lowers thecontrast ratio of the display, use of materials that have long-termreliability problems, and complex manufacturing processes that increasethe expense of the product.

[0007] Many prior art devices include substantial non-reflective areason their surfaces. This provides low fill ratios, and provides lowerthan optimum reflective efficiency. For example, U.S. Pat. No. 4,229,732discloses MOSFET devices that are formed on the surface of a device inaddition to mirrors. These MOSFET devices take up surface area, reducingthe fraction of the device area that is optically active and reducingreflective efficiency. The MOSFET devices on the surface of the devicealso diffract incident light, which lowers the contrast ratio of thedisplay. Further, intense light striking exposed MOSFET devicesinterfere with the proper operation of the devices, both by charging theMOSFET devices and overheating the circuitry.

[0008] Some SLM designs have rough surfaces, which also reducereflective efficiency. For example, in some SLM designs the reflectivesurface is an aluminum film deposited on an LPCVD silicon nitride layer.It is difficult to control the smoothness of these reflective mirrorsurfaces as they are deposited thin films. Thus, the final product hasrough surfaces, which reduce the reflective efficiency.

[0009] Another problem that reduces reflective efficiency with some SLMdesigns, particularly in some top hanging mirror designs, is largeexposed hinge surface areas. These large exposed hinge surface areashave to be blocked by a slab, typically made of tungsten, on top of thehinge to prevent the scattering of incident light. These slabssignificantly reduce the optically active area and lower the reflectiveefficiency.

[0010] Many conventional SLMs, such as the SLM disclosed in U.S. Pat.No. 4,566,935, have hinges made of aluminum alloy. Aluminum, as well asother metals, is susceptible to fatigue and plastic deformation, whichcan lead to long-term reliability problems. Also, aluminum issusceptible to cell “memory”, where the rest position begins to tilttowards its most frequently occupied position. Further, the mirrorsdisclosed in the U.S. Pat. No. 4,566,935 are released by undercuttingthe mirror surface. This technique often results in breakage of thedelicate micro-mirror structures during release. It also requires largegaps between mirrors, which reduce the fraction of the device area thatis optically active.

[0011] What is desired is an SLM with improved reflective efficiency,SLM device long-term reliability, and simplified manufacturingprocesses.

SUMMARY OF THE INVENTION

[0012] The present invention is a spatial light modulator (SLM). In oneembodiment, the SLM has a reflective selectively deflectable micromirror array fabricated from a first substrate bonded to a secondsubstrate having individually addressable electrodes. The secondsubstrate may also have addressing and control circuitry for the micromirror array. Alternatively, portions of the addressing and controlcircuitry are on a separate substrate and connected to the circuitry andelectrodes on the second substrate.

[0013] The micro mirror array includes a controllably deflectable mirrorplate with a reflective surface to reflect incident light. The mirrorplate is connected to a vertical hinge by a connector, and the hinge isin turn connected to spacer walls by support posts. Each of the mirrorplate, the connector, the vertical hinge, the support posts, and thespacer walls is fabricated from a first substrate. This first substrateis a wafer of a single material, single crystal silicon in oneembodiment. The spacer walls provide separation between the mirror plateand an electrode associated with that mirror plate that controls thedeflection of the mirror plate, and is located on the second substratebonded to the micro mirror array. The close spacing of the mirror platesand the vertical orientation of the hinge allow the reflective surfacesto have a very high fill ratio for the micro mirror array. Very littlelight gets past the micro mirror array to strike the circuitry on thesecond substrate.

[0014] The spatial light modulator is fabricated with few steps, whichkeeps the fabrication cost and complexity low. Cavities are formed in afirst side of the first substrate. This is done in a single anisotropicetch in one embodiment. In parallel, the electrodes and addressing andcontrol circuitry are fabricated on a first side of the secondsubstrate. The first side of the first substrate is bonded to the firstside of the second substrate. The sides are aligned so the electrodes onthe second substrate are in proper relation with the mirror plates whichthe electrodes will control. The second side of the first substrate isthinned to the desired thickness. Optionally, a layer of reflectivematerial may be deposited on a second side of the first substrate. Asecond anisotropic etch defines the support posts, the vertical hinges,and the connectors, and releases the mirror plates from the second sideof the first substrate. Thus, the spatial light modulator may befabricated by only two main etch steps.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a diagram that illustrates the general architecture of aspatial light modulator according to one embodiment of the invention.

[0016]FIGS. 2a and 2 b are perspective views of a single micro mirror.

[0017]FIGS. 3a and 3 b are perspective views showing the top and sidesof a micro mirror array.

[0018]FIGS. 4a and 4 b are perspective views showing the bottom andsides of the micro mirror array.

[0019]FIGS. 5a and 5 b are top views of the micro mirror array.

[0020]FIGS. 6a and 6 b are bottom views of the micro mirror array.

[0021]FIGS. 7a-7 d are perspective views showing the top, bottom, andsides of a single mirror of an alternate embodiment of the micro mirrorarray.

[0022]FIGS. 8a-8 d are perspective views showing the top and bottom ofthe alternate micro mirror array.

[0023]FIG. 9a is a flowchart illustrating a preferred embodiment of howthe spatial light modulator is fabricated.

[0024]FIGS. 9b through 9 j are block diagrams illustrating thefabrication of the spatial light modulator in more detail.

[0025]FIG. 10 illustrates the generation of the mask and the etchingthat forms the cavities in the first substrate in more detail.

[0026]FIG. 11 is a perspective view of one embodiment of the electrodesformed on the second substrate.

[0027]FIG. 12 is a perspective view showing the micro mirror array onthe first substrate positioned over the electrodes and other circuitryon the second substrate.

[0028]FIG. 13 illustrates a simplified embodiment of a mask that is usedin etching the upper surface of the first substrate.

[0029]FIG. 14 is a cross-section of a portion of the two substratesbonded together.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The reflective spatial light modulator (“SLM”) 100 has an array103 of deflectable mirrors 202. Individual mirrors 202 can beselectively deflected by applying a voltage bias between that mirror anda corresponding electrode 126. The deflection of each mirror 202controls light reflected from a light source to a video display. Thus,controlling the deflection of a mirror 202 allows light striking thatmirror 202 to be reflected in a selected direction, and thereby allowscontrol of the appearance of a pixel in the video display.

[0031] Spatial Light Modulator Overview:

[0032]FIG. 1 is a diagram that illustrates the general architecture ofan SLM 100 according to one embodiment of the invention. The illustratedembodiment has three layers. The first layer is a mirror array 103 thathas a plurality of deflectable micro mirrors 202. In one preferredembodiment, the micro-mirror array 103 is fabricated from a firstsubstrate 105 that is a single material, such as single crystal silicon.

[0033] The second layer is an electrode array 104 with a plurality ofelectrodes 126 for controlling the micro-mirrors 202. Each electrode 126is associated with a micro-mirror 202 and controls the deflection ofthat micro-mirror 202. Addressing circuitry allows selection of a singleelectrode 126 for control of the particular micro-mirror 202 associatedwith that electrode 126.

[0034] The third layer is a layer of control circuitry 106. This controlcircuitry 106 has addressing circuitry, which allows the controlcircuitry 106 to control a voltage applied to selected electrodes 126.This allows the control circuitry 106 to control the deflections of themirrors 202 in the mirror array 103 via the electrodes 126. Typically,the control circuitry 106 also includes a display control 108, linememory buffers 110, a pulse width modulation array 112, and inputs forvideo signals 120 and graphics signals 122. A microcontroller 114,optics control circuitry 116, and a flash memory 118 may be externalcomponents connected to the control circuitry 106, or may be included inthe control circuitry 106 in some embodiments. In various embodiments,some of the above listed parts of the control circuitry 106 may beabsent, may be on a separate substrate and connected to the controlcircuitry 106, or other additional components may be present as part ofthe control circuitry 106 or connected to the control circuitry 106.

[0035] In one embodiment, both the second layer 104 and the third layer106 are fabricated using semiconductor fabrication technology on asingle second substrate 107. That is, the second layer 104 is notnecessarily separate and above the third layer 106. Rather, the term“layer” is an aid for conceptualizing different parts of the spatiallight modulator 100. For example, in one embodiment, both the secondlayer 104 of electrodes is fabricated on top of the third layer ofcontrol circuitry 106, both fabricated on a single second substrate 107.That is, the electrodes 126, as well as the display control 108, linememory buffers 110, and the pulse width modulation array 112 are allfabricated on a single substrate in one embodiment. Integration ofseveral functional components of the control circuitry 106 on the samesubstrate provides an advantage of improved data transfer rate overconventional spatial light modulators, which have the display control108, line memory buffers 110, and the pulse width modulation array 112fabricated on a separate substrate. Further, fabricating the secondlayer of the electrode array 104 and the third layer of the controlcircuitry 106 on a single substrate 107 provides the advantage of simpleand cheap fabrication, and a compact final product.

[0036] After the layers 103, 104, and 106 are fabricated, they arebonded together to form the SLM 100. The first layer with the mirrorarray 103 covers the second and third layers 104, 106. The area underthe mirrors 202 in the mirror array 103 determines how much room thereis beneath the first layer 103 for the electrodes 126, and addressingand control circuitry 106. There is limited room beneath the micromirrors 202 in the mirror array 103 to fit the electrodes 126 and theelectronic components that form the display control 108, line memorybuffers 110, and the pulse width modulation array 112. The presentinvention uses fabrication techniques (described more fully below) thatallow the creation of small feature sizes, such as processes that allowfabrication of features of 0.18 microns, and processes that allow thefabrication of features of 0.13 microns or smaller. Conventional spatiallight modulators are made through fabrication processes that do notallow such small features. Typically, conventional spatial lightmodulators are made through fabrication processes that limit featuresize to approximately 1 micron or larger. Thus, the present inventionallows the fabrication of many more circuit devices, such astransistors, in the limited area beneath the micro mirrors of the mirrorarray 103. This allows integration of items such as the display control108, line memory buffers 110, and the pulse width modulation array 112on the same substrate as the electrodes 126. Including such controlcircuitry 106 on the same substrate 107 as the electrodes 126 improvesthe performance of the SLM 100.

[0037] In other embodiments, various combinations of the electrodes 126and components of the control circuitry may be fabricated on differentsubstrates and electrically connected.

[0038] The Mirror:

[0039]FIG. 2a is a perspective view of a single micro mirror 202. In onepreferred embodiment, the micro mirror 202 is fabricated from a wafer ofa single material, such as single crystal silicon. Thus, the firstsubstrate 105 in such an embodiment is a wafer of single crystalsilicon. Fabricating the micro mirror 202 out of a single material wafergreatly simplifies the fabrication of the mirror 202. Further, singlecrystal silicon can be polished to create smooth mirror surfaces thathave an order of magnitude smoother surface roughness than those ofdeposited films. Mirrors 202 fabricated from single crystal silicon aremechanically rigid, which prevents undesired bending or warping of themirror surface, and hinges fabricated from single crystal silicon aredurable, flexible, and reliable. In other embodiments, other materialsmay be used instead of single crystal silicon. One possibility is theuse of another type of silicon (e.g. polysilicon, or amorphous silicon)for the micro mirror 202, or even making the mirror 202 completely outof a metal (e.g. an aluminum alloy, or tungsten alloy).

[0040] The micro mirror 202 has a top mirror plate 204. This mirrorplate 204 is the portion of the micro mirror 202 that is selectivelydeflected by applying a voltage bias between the mirror 202 and acorresponding electrode 126. In one embodiment this reflective mirrorplate 204 is substantially square in shape, and approximately fifteenmicrons by fifteen microns, for an approximate area of 225 squaremicrons, although other shapes and sizes are also possible. In onepreferred embodiment, a large proportion of the surface area of themicro mirror array 103 is made up of the areas of the mirror plates 204of the micro mirrors 202.

[0041] The mirror plate 204 has a reflective surface that reflects lightfrom a light source at an angle determined by the deflection of themirror plate 204. This reflective surface may be the same material fromwhich the micro mirror 202 is fabricated, in which case the surface ofthe mirror plate 204 is polished to a smoothness that provides thedesired level of reflectivity. Alternatively, after fabrication of themicro-mirrors 202, a layer of reflective material, such as aluminum, maybe added to the surface of the mirror plate 204. Since in a preferredembodiment a large proportion of the surface area of the micro mirrorarray 103 is made up of the areas of the mirror plates 204 of the micromirrors, and the mirror plates 204 have reflective surfaces, a largeproportion of the surface area of the micro mirror array 103 isreflective and capable of reflecting light at a selected angle. Thus,the SLM 100 has a large fill ratio, and efficiently reflects incidentlight.

[0042] The mirror plate 204 is connected to a torsion spring hinge 206by a connector 216. The torsion spring hinge 206 is connected to aspacer support frame 210, which holds the torsion spring 206 in place.Note that other springs and connection schemes between the mirror plate204, the hinge 206, and spacer support frame 210 could also be used. Thetorsion spring hinge 206 allows the mirror plate 204 to rotate relativeto the spacer support frame 210 about an axis between the walls of thespacer support frame 210 when a force such as an electrostatic force isapplied to the mirror plate 204 by applying a voltage between the mirror202 and the corresponding electrode 126. This rotation produces theangular deflection for reflecting light in a selected direction. In oneembodiment, this rotation occurs about an axis that is substantiallycollinear with the long axis of the hinge. In one preferred embodiment,the torsion spring hinge 206 has a “vertical” alignment. That is, thehinge 206 has a width 222 that is smaller than the depth of the hinge(perpendicular to the mirror plate 204 surface). The width of the hingeis typically between 0.1 microns to 0.5 microns, and is approximately0.2 microns in one embodiment. This “vertical” alignment of the hingefunctions to help minimize non-reflective surfaces on the surface of themirror array 103, and keep the fill ratio high. Also in one preferredembodiment, the

[0043] The spacer support frame 210 separates the mirror plate 204 fromthe electrodes and addressing circuitry so that the mirror plate 204 maydeflect downward without contacting the electrodes and other circuitrybelow. The spacer support frame 210 includes spacer walls in oneembodiment, which are typically not separate components from the rest ofthe spacer support frame 210. These walls help define the height of thespacer support frame 210. The height of the spacers 210 is chosen basedon the desired separation between the mirror plates 204 and theelectrodes 126, and the topographic design of the electrodes. A largerheight allows more deflection of the mirror plate 204, and a highermaximum deflection angle. A larger deflection angle provides a bettercontrast ratio. In one embodiment, the maximum deflection angle of themirror plate 204 is 20 degrees. The spacer support frame 210 alsoprovides support for the hinge 206 and spaces the mirror plate 204 fromother mirror plates 204 in the mirror array 103. The spacer supportframe 210 has a spacer wall width 212, which, when added to a gapbetween the mirror plate 204 and the support frame 210, is substantiallyequal to the distance between adjacent mirror plates 204 of adjacentmicro mirrors 202. In one embodiment, the spacer wall width 212 is 1micron or less. In one preferred embodiment, the spacer wall width 212is 0.5 microns or less. This places the mirror plates 204 closelytogether to increase the fill ratio of the mirror array 103.

[0044] In some embodiments, the micro mirror 202 includes elements thatstop the deflection of the mirror plate 204 when the plate 204 hasdeflected downward to a predetermined angle. Typically, these elementsinclude a motion stop and a landing tip. When the mirror surface 204deflects, the motion stop on the mirror plate 204 contacts the landingtip. When this occurs, the mirror plate 204 can deflect no further.There are several possible configurations for the motion stop andlanding tip. In one embodiment, a landing tip is fabricated on thespacer frames 210 opposite to the hinge side. The maximum tilt angle ofmirror plate 204 will be limited by the landing tip on the spacer frames210 which stops the downward mechanical motion of the mirror plate 204.Having a fixed maximum tilt angle simplifies controlling the spatiallight modulator 100 to reflect incident light in a known direction.

[0045] In another embodiment, landing tips are fabricated along with theelectrodes 126 on the second substrate 107. The landing tips of thisembodiment may be fabricated from an insulator, such as silicon dioxide,to prevent a short circuit between the mirror plate 204 and theelectrode 126. The maximum tilt angle of the mirror plate 204 is limitedin this embodiment by the angle at which the mirror plate 204 contactsthe landing tip on the second substrate 107. The height of the spacers210 affects this angle; higher spacers 210 allow larger angles thanlower ones. The landing tip on the second substrate 107 can be aprotruding bump, which reduces the total surface area actually incontact. The bumps can be held at the same electrical potential as themirror plate 204 to avoid welding on contact.

[0046] In yet another embodiment, the gap between the mirror plate 204and the hinge 206 is accurately fabricated so when the mirror plate 204tilts to a predetermined angle, the corners of the plate 204 near thehinge 206 will contact the ends of the hinge 206, which act asmechanical stops. This occurs because the section of the hinge 206connected to the mirror plate 204 deflects along with the mirror plate204, but the sections of the hinge 206 near the support wall 210 remainrelatively undeflected. For example, with a height of the torsion hinge206 being 0 micron, a gap of 0.13 microns between the support wall andthe hinge 206 will result in a maximum tilting angle of the mirror plate204 of 15 degrees.

[0047] In one preferred embodiment, the motion stop and landing tip areboth made out of the same material as the rest of the mirror 202, andare both fabricated out of the first substrate 105. In embodiments wherethe material is single crystal silicon, the motion stop and landing tipare therefore made out of a hard material that has a long functionallifetime, which allows the mirror array 103 to last a long time.Further, because single crystal silicon is a hard material, the motionstop and landing tip can be fabricated with a small area where themotion stop contacts the landing tip, which greatly reduces stickingforces and allows the mirror plate 204 to deflect freely. Also, thismeans that the motion stop and landing tip remain at the same electricalpotential, which prevents sticking that would occur via welding andcharge injection processes were the motion stop and landing tip atdifferent electrical potentials.

[0048]FIG. 2b is a perspective view illustrating the underside of asingle micro mirror 202, including the support walls 210, the mirrorplate 204, the hinge 206, and the connector 216.

[0049]FIG. 3a is a perspective view showing the top and sides of a micromirror array 103 having nine micro mirrors 202-1 through 202-9. WhileFIG. 3a shows the micro mirror array 103 with three rows and threecolumns, for a total of nine micro mirrors 202, micro mirror arrays 103of other sizes are also possible. Typically, each micro mirror 202corresponds to a pixel on a video display. Thus, larger arrays 103 withmore micro mirrors 202 provide a video display with more pixels. Sincethe hinges 206 in the mirror array 103 all face in parallel along onedirection, light sources are directed at the mirrors 202 in the array103 along a single direction to be reflected to form a projected imageon the video display.

[0050] As shown in FIG. 3a, the surface of the micro mirror array 103has a large fill ratio. That is, most of the surface of the micro mirrorarray 103 is made up of the reflective surfaces of the mirror plates 204of the micro mirrors 202. Very little of the surface of the micro mirrorarray 103 is nonreflective. As illustrated in FIG. 3a, the nonreflectiveportions of the micro mirror array 103 surface are the areas between thereflective surfaces of the micro mirrors 202. For example, the width ofthe area between mirror 202-1 and 202-2 is determined by the spacer wallwidth 212 and the sum of the width of the gaps between the mirror plates204 of mirrors 202-1 and 202-2 and the support wall 210. The gaps andthe spacer wall width 212 can be made as small as the feature sizesupported by the fabrication technique. Thus, in one embodiment, thegaps are 0.2 micron, and in another embodiment the gaps are 0.13 micron.As semiconductor fabrication techniques allow smaller features, the sizeof the spacer wall 210 and the gaps can decrease to allow higher fillratios. FIG. 3b is a perspective view detailing one mirror 202 of themirror array 103 of FIG. 3a. Embodiments of the present invention allowfill ratios of 85%, 90%, or even higher.

[0051]FIG. 4a is a perspective view showing the bottom and sides of themicro mirror array 103 shown in FIG. 3. As shown in FIG. 4a, the spacersupport frames 210 of the micro mirrors 202 define cavities beneath themirror plates 204. These cavities provide room for the mirror plates 204to deflect downwards, and also allow large areas beneath the mirrorplates 204 for placement of the second layer 104 with the electrodes126, and/or the third layer with the control circuitry 106. FIG. 4b is aperspective view detailing one mirror 202 of the mirror array 103 ofFIG. 4a.

[0052]FIG. 5a is a top view of the micro mirror array 103 having ninemicro mirrors 202-1 through 202-9 shown in FIGS. 3a and 4 a. Forexample, for micro mirror 202-1, FIG. 5a illustrates the mirror plate204, the spacer support frame 210, the torsion spring 206, and theconnector 216 connecting the mirror plate 204 to the torsion spring 206.FIG. 5a also clearly illustrates, as described above with respect toFIG. 3a, that the micro mirror array 103 has a large fill ratio. Most ofthe surface of the micro mirror array 103 is made up of the reflectivesurfaces of the micro mirrors 202-1 through 202-9. FIG. 5a clearlyillustrates how fill ratio is determined by the areas of the reflectivemirror plates 204 and the areas between the reflective surfaces of themirror plates 204. The size of the areas between the reflective surfacesof the mirror plates 204 in one embodiment is limited by the featuresize limit of the fabrication process. This determines how small thegaps between the mirror plate 204 and the spacer wall 210 can be made,and how thick the spacer wall 210 is. Note that, while the single mirror202 as shown in FIG. 2 has been described as having its own spacersupport frame 210, there are not typically two separate abutting spacerwalls 210 between mirrors such as mirrors 202-1 and 202-2. Rather, thereis typically one physical spacer wall of the support frame 210 betweenmirrors 202-1 and 202-2. FIG. 5b is a perspective view detailing onemirror 202 of the mirror array 103 of FIG. 5a.

[0053]FIG. 6a is a bottom view of the micro mirror array 103 having ninemicro mirrors 202-1 through 202-9, as shown in FIGS. 3 through 5. FIG.6a shows the bottom of the mirror plates 204, as well as the bottoms ofthe spacer support frames 210, the torsion springs 206, and theconnectors 216. The area beneath the mirror plates 204 is large enoughin many embodiments to allow the optimum design and placement ofelectrodes 126 and control circuitry 106, and space for accommodating apossible mirror landing tip. FIG. 6b is a perspective view detailing onemirror 202 of the mirror array 103 of FIG. 6a

[0054] As seen in FIGS. 5a and 6 a, very little light that is normal tothe mirror plate 204 can pass beyond the micro mirror array 103 to reachany the electrodes 126 or control circuitry 106 beneath the micro mirrorarray 103. This is because the spacer support frame 210, the torsionspring 206, the connector 216, and the mirror plate 204 provide nearcomplete coverage for the circuitry beneath the micro mirror array 103.Also, since the spacer support frame 210 separates the mirror plate 204from the circuitry beneath the micro mirror array 103, light travelingat a non perpendicular angle to the mirror plate 204 and passing beyondthe mirror plate 204 is likely to strike a wall of the spacer supportframe 210 and not reach the circuitry beneath the micro mirror array103. Since little intense light incident on the mirror array 103 reachesthe circuitry, the SLM 100 avoids problems associated with intense lightstriking the circuitry. These problems include the incident lightheating up the circuitry, and the incident light photons chargingcircuitry elements, both of which can cause the circuitry tomalfunction.

[0055] In FIGS. 3-6 each micro mirror 202 in the micro mirror array 103has its torsion spring 206 on the same side. In one alternateembodiment, different micro mirrors 202 in the micro mirror array 103have torsion springs 206 on different sides. For example, returning toFIG. 3a, mirrors 202-1 and 202-3 would have springs 206 on the same sideas illustrated. Mirror 202-2, in contrast, would have a spring 206 on adifferent side so that the spring 206 of mirror 202-2 is perpendicularto the springs 206 of mirrors 202-1 and 202-3. This allows the mirrorplates 204 of the different micro mirrors 202-1 and 202-2 to deflect indifferent directions, which gives the mirror array 103 as a whole morethan one controllable degree of freedom. In this alternate embodiment,two different light sources (for example, light sources with differentlycolored light) can be directed toward the micro mirror array 103 andseparately selectively redirected by the micro mirrors 202 in the micromirror array 103 form an image on a video display. In such anembodiment, multiple micro mirrors 202 can be used to reflect light fromthe multiple light sources to the same pixel in the video display. Forexample, two different color light sources can be directed toward themirror array 103 along different directions, and reflected by the array103 to form a multicolor image on a video display. The micro mirrors202-1 and 202-3 with torsion springs 206 on a first side control thereflection of a first light source to the video display. The micromirrors such as micro mirror 202-2 with torsion springs 206 on adifferent second side control the reflection of a second light source tothe video display.

[0056]FIG. 7a is a perspective view of a micro mirror 702 according toan alternate embodiment of the invention. The torsion hinge 206 in thisembodiment is diagonally oriented with respect to the spacer supportwall 210, and divides the mirror plate 204 into two parts, or sides: afirst side 704 and a second side 706. Two electrodes 126 are associatedwith the mirror 702, one electrode 126 for a first side 704 and oneelectrode 126 for a second side 706. This allows either side 704, 706 tobe attracted to one of the electrodes 126 beneath and pivot downward,and provides more total range of angular motion for the same supportwall 210 height as compared to the mirror illustrated in FIGS. 2-6. FIG.7b is a more detailed view of the mirror 702 and illustrates the mirrorplate 204, hinge 206, and support wall 210. FIGS. 7c and 7 d illustratethe underside of a single mirror 702 and a more detailed view of theinterior corner of the mirror 702. In other embodiments, the hinge 206may be substantially parallel to one of the sides of the mirror plate204, rather than diagonal, and still be positioned to divide the mirrorplate 204 into two parts 704, 706.

[0057]FIGS. 8a through 8 d are various perspective views of mirrorarrays composed of multiple micro mirrors 702 as described in FIGS. 7athrough 7 d. FIGS. 8a and 8 b illustrate the top of a mirror 702 arrayand a more detailed view of one mirror 702 in the array. FIGS. 8c and 8d illustrate the underside of a mirror 702 array and a more detailedview of one mirror 702 in the array.

[0058] Fabrication of the Spatial Light Modulator:

[0059]FIG. 9a is a flowchart illustrating one preferred embodiment ofhow the spatial light modulator 100 is fabricated. FIGS. 9b through 9 gare block diagrams illustrating the fabrication of the spatial lightmodulator 100 in more detail. In summary, the micro mirrors 202 arepartially fabricated on the first substrate 105. Separately, some or allof the electrodes, addressing circuitry, and control circuitry arefabricated on the second substrate 107. The first and second substrates105 and 107 are then bonded together. The first substrate 105 isthinned, then lithography and etch steps follow. Then the fabrication ofthe micro mirrors 202 is completed. Final steps, including packaging,complete the spatial light modulator 100. In one embodiment, the mirrorarray 103 is fabricated from a wafer of single crystal silicon usingonly anisotropic dry etch methods, only two etches are done to fabricatethe mirror array 103, and the circuitry is fabricated using standardCMOS techniques. This provides an easy and inexpensive way to fabricatethe SLM 100.

[0060] Conventional spatial light modulators are fabricated with surfacemicro machining techniques that include etching, deposition ofstructural layers, deposition and removal of sacrificial layers. Theseconventional MEMS fabrication techniques result in poor yield, pooruniformity, and result feature sizes of approximately 1 micron orlarger. In contrast, one embodiment of the present invention usessemiconductor fabrication techniques, which do not include sacrificiallayers, have much higher yields, and allow creation of features of 0.13microns or smaller.

[0061] Referring to FIG. 9a, a first mask is generated 902 to initiallypartially fabricate the micro mirrors 202. This mask defines what willbe etched from one side of the first substrate 105 to form the cavitieson the underside of the micro mirror array 103 that define the spacersupport frames 210 and support posts 208. Standard techniques, such asphotolithography, can be used to generate the mask on the firstsubstrate. As mentioned previously, in one preferred embodiment themicro mirrors 202 are formed from a single material, such as singlecrystal silicon. Thus, in one preferred embodiment, the first substrate105 is a wafer of single crystal silicon. Note that typically multiplemicro mirror arrays 103, to be used in multiple SLMs 100, are fabricatedon a single wafer, to be separated later. The structures fabricated tocreate the micro mirror array 103 are typically larger than the featuresused in CMOS circuitry, so it is relatively easy to form the micromirror array 103 structures using known techniques for fabricating CMOScircuitry. FIG. 9b is a side view that illustrates the first substrate105 prior to fabrication. The substrate 105 initially includes a devicelayer 938, which is the material from which the mirror array 103 will befabricated, an insulating oxide layer 936, and a handling substrate 934.FIG. 9c is a side view that illustrates the first substrate 105 with themask upon it.

[0062] After the mask is generated 902, in a preferred embodiment, thefirst substrate 105 is anisotropically ion etched 904 to form thecavities beneath the mirror plates 204. Put in another way, a “well” isformed in the first substrate for every micro mirror 202. Other methodsbesides an anisotropic ion etch may also be used to form the cavities or“wells,” such as a wet etch or a plasma etch. FIG. 9d is a block diagramthat shows the first substrate 105 with the cavities etched.

[0063] Separately from the fabrication of the cavities beneath themirror plates 204, the electrodes 126 and control circuitry 106 arefabricated 906 on the second substrate 107. The second substrate 107 maybe a transparent material, such as quartz, or another material. If thesecond substrate is quartz, transistors may be made from polysilicon, ascompared to crystalline silicon. The circuitry can be fabricated 906using standard CMOS fabrication technology. For example, in oneembodiment, the control circuitry 106 fabricated 906 on the secondsubstrate 107 includes an array of memory cells, row address circuitry,and column data loading circuitry. There are many different methods tomake electrical circuitry that performs the addressing function. TheDRAM, SRAM, and latch devices commonly known may all perform theaddressing function. Since the mirror plate 204 area may be relativelylarge on semiconductor scales (for example, the mirror plate 204 mayhave an area of 225 square microns), complex circuitry can bemanufactured beneath micro mirror 202. Possible circuitry includes, butis not limited to, storage buffers to store time sequential pixelinformation, circuitry to compensate for possible non-uniformity ofmirror plate 204 to electrode 126 separation distances by driving theelectrodes 126 at varying voltage levels, and circuitry to perform pulsewidth modulation conversions.

[0064] This control circuitry 106 is covered with a passivation layersuch as silicon oxide or silicon nitride. Next, a metallization layer isdeposited. This metallization layer is patterned and etched to defineelectrodes 126, as well as a bias/reset bus in one embodiment. Theelectrodes 126 are placed during fabrication so that one or more of theelectrodes 126 corresponds to each micro mirror 202. As with the firstsubstrate 105, typically multiple sets of circuitry to be used inmultiple SLMs 100 are fabricated 906 on the second substrate 107 to beseparated later.

[0065] Next, the first and second substrates are bonded 910 together.The side of the first substrate 105 that has the cavities is bonded tothe side of the second substrate 107 that has the electrodes. Thesubstrates 105 and 107 are aligned so that the electrodes on the secondsubstrate 107 are in the proper position to control the deflection ofthe micro mirrors 202 in the micro mirror array 103. In one embodiment,the two substrates 105 and 107 are optically aligned using doublefocusing microscopes by aligning a pattern on the first substrate 105with a pattern on the second substrate 107, and the two substrates 105and 107 are bonded together by low temperature bonding methods such asanodic or eutectic bonding. There are many possible alternateembodiments to the fabrication 906. For example, thermoplastics ordielectric spin glass bonding materials can be used, so that thesubstrates 105 and 107 are bonded thermal-mechanically. FIG. 9e is aside view that shows the first and second substrates 105, 107 bondedtogether.

[0066] After bonding the first and second substrates 105 and 107together, the surface of the first substrate 105 that has not beenetched is thinned 912 to a desired thickness. First, the handlingsubstrate 934 is removed, as shown in FIG. 9f, typically by grinding oretching. Then the oxide 936 is removed. Then, the device layer 938 isthinned or polished, if necessary. This thinning is done in oneembodiment by mechanical grinding the substrate 105 to a thicknessbetween the bottom of the fabricated “well” and the opposing surface ofthe first substrate 105 that is near the desired thickness of the micromirror 202. In one embodiment, this thickness achieved by mechanicalgrinding is approximately 5 microns. The substrate 105 is then polishedby mechanical fine polishing or chemical mechanical polishing tothickness desired between the bottom of the “well” and the opposingsurface of the first substrate 105. This thickness defines the thicknessof the mirror plates 204. In one embodiment, this desired thickness isless than approximately 1 micron or less. FIG. 9g is a side view showingthe bonded first and second substrates 105, 107 after the firstsubstrate 105 has been thinned.

[0067] Next, the reflective surface of the micro mirror 202 is created.This can be done through polishing 913 the first substrate 105 so thatthe surface of the first substrate 105 is reflective. It is alsopossible to deposit 914 a layer of a reflective material on the firstsubstrate 105 to create a reflective surface. Other methods to create areflective surface may also be used.

[0068] In one embodiment, a reflective layer of aluminum is deposited914. The thinned surface of the first substrate 105 is coated withapproximately 10 nm of titanium seed thin film. Then an approximately 30nm thick layer of aluminum is deposited to form a reflective layer witha reflectivity above 95% over much of the visible optical spectrum. FIG.9h is a side view that shows a deposited reflective layer 932.

[0069] The reflective surface of the first substrate 105 is then maskedand, in a preferred embodiment, high-aspect-ratio anisotropically ionetched 916 to finish forming the micro mirror array 103 and release themirror plates 204. This second etch defines the mirror plate 204, thetorsion spring hinge 206, and the connector 216. Thus, it only takes twoetchings of the first substrate 105 to fabricate the micro mirrors 202.This significantly decreases the cost of fabricating the micro mirrors202. FIG. 9i is a block diagram showing the surface of the firstsubstrate 105 covered with the mask 933, and FIG. 9j is a block diagramshowing the spatial light modulator 100 after the second etching,including the mirror plate 204, the hinge 206, the spacer support frame210, and the electrode 126.

[0070] In some embodiments, the hinges 206 are partially etched to berecessed from the surface of the mirror plates 204. Also, in someembodiments a reflective surface is deposited 914 after the second etchthat defines the mirror plate 204, the torsion spring hinge 206, and theconnector 216. Such a reflective layer may be deposited by, for example,evaporating aluminum downwardly at an angle such that the horizontalvector of the angle is from mirror plate 204 to hinges 206. With thisangle, and if the hinges 206 were etched so that they are recessed fromthe surface of the mirror plates 204, it is possible to depositsubstantially no reflective coating on the surfaces of recessed hinges206 to minimize the optical scattering of incident light by the surfacesof the torsion hinges 206. The evaporation may occur, for example, inthe reaction chamber of an e-gun thermal evaporator at a deposition rateof one nanometer per second.

[0071] In some embodiments, the micro-mirror array 103 is protected by apiece of glass or other transparent material. In one embodiment, duringfabrication of the micro mirror array 103, a rim is left around theperimeter of each micro mirror array 103 fabricated on the firstsubstrate 105. To protect the micro mirrors 202 in the micro mirrorarray 103, a piece of glass or other transparent material is bonded 918to the rim. This transparent material protects the micro mirrors 202from physical harm. In one alternative embodiment, lithography is usedto produce an array of rims in a layer of photosensitive resin on aglass plate. Then epoxy is applied to the upper edge of the rims, andthe glass plate is aligned and attached to the completed reflective SLM100.

[0072] As discussed above, multiple spatial light modulators 100 may befabricated from the two substrates 105 and 107; multiple micro mirrorarrays 103 may be fabricated in the first substrate 105 and multiplesets of circuitry may be fabricated in the second substrate 107.Fabricating multiple SLMs 100 increases the efficiency of the spatiallight modulator 100 fabrication process. However, if multiple SLMs 100are fabricated at once, they must be separated into the individual SLMs100. There are many ways to separate each spatial light modulator 100and ready it for use. In a first method, each spatial light modulator100 is simply die separated 920 from the rest of the SLMs 100 on thecombined substrates 105 and 107. Each separated spatial light modulator100 is then packaged 922 using standard packaging techniques.

[0073] In a second method, a wafer-level-chip-scale packaging is carriedout to encapsulate each SLM 100 into separate cavities and formelectrical leads before the SLMs 100 are separated. This furtherprotects the reflective deflectable elements and reduces the packagingcost. In one embodiment of this method, the backside of the secondsubstrate 107 is bonded 924 with solder bumps. The backside of thesecond substrate 107 is then etched 926 to expose metal connectors thatwere formed during fabrication of the circuitry on the second substrate107. Next, conductive lines are deposited 928 between the metalconnectors and the solder bumps to electrically connect the two.Finally, the multiple SLMs are die separated 930.

[0074]FIG. 10 illustrates the generation 902 of the mask 1000 and theetching 904 that forms the cavities in the first substrate in moredetail. In a preferred embodiment, the first substrate is a wafer ofsingle crystal silicon. Oxide is deposited and patterned on the firstsubstrate. This results in the pattern shown in FIG. 10, where area 1004is oxide that will prevent the substrate beneath from being etched, andareas 1002 are areas of exposed substrate. The areas of exposedsubstrate 1002 will be etched to form the cavities. The areas 1004 thatare not etched remain, and form the spacer support posts 208 and thespacer support frame 210.

[0075] In one embodiment, the substrate is etched in a reactive ion etchchamber flowing with SF6, HBr, and oxygen gases at flow rates of 100sccm, 50 sccm, and 10 sccm respectively. The operating pressure is inthe range of 10 to 50 mTorr, the bias power is 60 W, and the sourcepower is 300 W. In another embodiment, the substrate is etched in areactive ion etch chamber flowing with Cl2, HBr, and oxygen gases atflow rates of 100 sccm, 50 sccm, and 10 sccm respectively. In theseembodiments, the etch processes stop when the cavities are about 3-4microns deep. This depth is measured using in-situ etch depthmonitoring, such as in-situ optical interferometer techniques, or bytiming the etch rate.

[0076] In another embodiment, the cavities are formed in the wafer by ananisotropic reactive ion etch process. The wafer is placed in a reactionchamber. SF6, HBr, and oxygen gases are introduced into the reactionchamber at a total flow rate of 100 sccm, 50 sccm, and 20 sccmrespectively. A bias power setting of 50 W and a source power of 150 Ware used at a pressure of 50 mTorr for approximately 5 minutes. Thewafers are then cooled with a backside helium gas flow of 20 sccm at apressure of 1 mTorr. In one preferred embodiment, the etch processesstop when the cavities are about 3-4 microns deep. This depth ismeasured using in-situ etch depth monitoring, such as in-situ opticalinterferometer techniques, or by timing the etch rate.

[0077]FIG. 11 is a perspective view of one embodiment of the electrodes126 formed on the second substrate 107. In this embodiment, each micromirror 202 has a corresponding electrode 126. The electrodes 126 in thisillustrated embodiment are fabricated to be higher than the rest of thecircuitry on the second substrate 107. As shown in FIG. 11, material onthe sides of the electrodes 126 slopes down from the electrodes' topsurface in a somewhat pyramid shape. In other embodiments, theelectrodes 126 are located on the same level as the rest of thecircuitry on the second substrate 107, rather than extending above thecircuitry. In one embodiment of the invention, the electrodes 126 areindividual aluminum pads of approximately 10×10 microns in size. Theseelectrodes 126 are fabricated on the surface of the second substrate107. The large surface area of the electrodes 126 in this embodimentresults in relatively low addressing voltages required to pull themirror plate 204 down onto the mechanical stops, to cause the fullpre-determined angular deflection of the mirror plates 204.

[0078]FIG. 12 is a perspective view showing the micro mirror array 103on the first substrate 105 positioned over the electrodes 126 and othercircuitry on the second substrate 107. This illustrates the relativepositions of the micro mirrors 202 in the micro mirror array 103 and theelectrodes prior to bonding 910 the first and second substrates 105 and107 together. Note that, for illustrative purposes, the micro mirrors202 in the micro mirror array 103 are shown as completed micro mirrors202. However, in a preferred embodiment, as described with respect toFIG. 9a, only the cavities beneath the mirror plates 204 in the firstsubstrate 105 would have been etched prior to bonding the firstsubstrate 105 to the second substrate 107. The mirror plate 204, hinges206, and connectors 216 would not be fabricated yet. In embodimentswhere the electrodes 126 are located above the level of the rest of thecircuitry and material on the side of the electrodes 126 slopes down,the sloping material helps correctly position the first substrate 105 onthe second substrate 107.

[0079]FIG. 13 illustrates a simplified embodiment of a mask that is usedin etching 916 the upper surface of the first substrate 105. In theetching 916 step, areas 1302 are left exposed and are etched to releasethe mirror plates 204 and form the torsion springs 206, the connectors216, and the support posts 208. Other areas 1304 are covered withphotoresist material and are not etched. These areas include the mirrorplates 204 themselves and the material that will form the hinges 206. Asshown in FIG. 13, most of the surface of the mirror array 103 isreflective. The fabrication process only creates small nonreflectivegaps that separate the mirror plates 204 from the support walls 210 andhinges 206.

[0080] The upper surface of the first substrate 105 is etched to releasethe mirror plates 204 and form the hinges 206 after the upper surface ofthe first substrate 105 is masked. In one embodiment, it is etched in areactive ion etch chamber flowing with SF6, HBr, and oxygen gases at aflow rate of 100 sccm, 50 sccm, and 10 sccm respectively. The operatingpressure is in the range of 10 to 50 mTorr, and the bias power of 60 Wand a source power 300 W. Since the etch depth is typically less than 1micron, there are several other fabrication processes can achieve thesame goal. Another embodiment uses Cl2 and oxygen gases at an operatingpressure of 10 mTorr to 50 mTorr with bias and source power settings ofthe etching reaction chamber of 50 W and 300 W, respectively, to achievetight dimension control. The etch process is stopped at the desireddepth (in one embodiment, about 5 microns deep) using in-situ etch depthmonitoring or by timing the etch rate.

[0081] Operation:

[0082] In operation, individual reflective elements are selectivelydeflected and serve to spatially modulate light that is incident to andreflected by the mirrors.

[0083]FIG. 14 is a cross-section that shows the micro mirror 202 abovean electrode 126. In operation, a voltage is applied to an electrode 126to control the deflection of the corresponding mirror plate 204 abovethe electrode 126. As shown in FIG. 14, when a voltage is applied to theelectrode 126, the mirror plate 204 is attracted to the electrode. Thiscauses the mirror plate 204 to rotate about the torsion spring 206. Whenthe voltage is removed from the electrode 126, the hinge 206 causes themirror plate 204 to spring back upward. Thus, light striking the mirrorplate 204 is reflected in a direction that can be controlled by theapplication of voltage to the electrode.

[0084] One embodiment is operated as follows. Initially the mirror plateis undeflected. In this unbiased state, an incoming light beam, from alight source, obliquely incident to SLM 100 is reflected by the flatmirror plates 204. The outgoing, reflected light beam may be receivedby, for example, an optical dump. The light reflected from theundeflected mirror plate 204 is not reflected to a video display.

[0085] When a voltage bias applied between the mirror plate 204 and thebottom electrode 126, the mirror plate 204 is deflected due toelectrostatic attraction. Because of the design of the hinge 206, thefree end of the mirror plate 204 is deflected towards the secondsubstrate 107. Note that in one preferred embodiment substantially allthe bending occurs in the hinge 206 rather than the mirror plate 204.This may be accomplished in one embodiment by making the hinge width 222thin, and connecting the hinge 206 to the support posts 208 only on bothends. The deflection of the mirror plate 204 is limited by motion stops,as described above. The full deflection of the mirror plate 204 deflectsthe outgoing reflected light beam into the imaging optics and to thevideo display.

[0086] When the mirror plate 204 deflects past the “snapping” or“pulling” voltage (approximately 12 volts in one embodiment), therestoring mechanical force or torque of the hinge 206 can no longerbalance the electrostatic force or torque and the mirror plate 204“snaps” down toward the electrode 126 to achieve full deflection,limited only by the motion stops. To release the mirror plate 204 fromits fully deflected position, the voltage must be lowered substantiallybelow the snapping voltage to a releasing voltage (e.g., approximately3.3 volts, in the embodiment where the snapping voltage is 5.0 volts).Thus, the micro mirror 202 is an electromechanically bistable device.Given a specific voltage between the releasing voltage and the snappingvoltage, there are two possible deflection angles at which the mirrorplate 204 may be, depending on the history of mirror plate 204deflection. Therefore, the mirror plate 204 deflection acts as a latch.These bistability and latching properties exist since the mechanicalforce required for deflection of the mirror plate 204 is roughly linearwith respect to deflection angle, whereas the opposing electrostaticforce is inversely proportional to the distance between the mirror plate204 and the electrode 126.

[0087] Since the electrostatic force between the mirror plate 204 andthe electrode 126 depends on the total voltage between the mirror plate204 and the electrode 126, a negative voltage applied to a mirror plate204 reduces the positive voltage needed to be applied to the electrode126 to achieve a given deflection amount. Thus, applying a voltage to amirror array 103 can reduce the voltage magnitude requirement of theelectrodes 126. This can be useful, for example, because in someapplications it is desirable to keep the maximum voltage that must beapplied to the electrodes 126 below 12V because a 5V switchingcapability is more common in the semiconductor industry. In addition,the amount of charge needed to bias each electrode 126 where a voltageis applied to a mirror array 103 is smaller than the charge needed in anembodiment in which the mirror array 103 is held at a ground potential.Thus the time required to correctly apply the proper voltage to theelectrode 126 and deflect the mirror plate 204 is relatively fast.

[0088] Since the maximum deflection of the mirror plate 204 is fixed,the SLM 100 can be operated in a digital manner if it is operated atvoltages past the snapping voltage. The operation is essentially digitalbecause the mirror plate 204 is either fully deflected downward byapplication of a voltage to the associated electrode 126 or is allowedto spring upward, with no voltage applied to the associated electrode126. A voltage that causes the mirror plate 204 to fully deflectdownward until stopped by the physical elements that stop the deflectionof the mirror plate 204 is known as a “snapping” or “pulling” voltage.Thus, to deflect the mirror plate 204 fully downward, a voltage equal orgreater to the snapping voltage is applied to the correspondingelectrode 126. In video display applications, when the mirror plate 204is fully deflected downward, the incident light on that mirror plate 204is reflected to a corresponding pixel on a video display. When themirror plate 204 is allowed to spring upward, the light is reflected insuch a direction so that it does not strike the video display.

[0089] During such digital operation, it is not necessary to keep thefull snapping voltage on an electrode 126 after an associated mirrorplate 204 has been fully deflected. During an “addressing stage,”voltages for selected electrodes 126 that correspond to the mirrorplates 204 which should be fully deflected are set to levels required todeflect the mirror plates 204. After the mirror plates 204 in questionhave deflected due to the voltages on electrodes 126, the voltagerequired to hold the mirror plates 204 in the deflected position is lessthan that required for the actual deflection. This is because the gapbetween the deflected mirror plate 204 and the addressing electrode 126is smaller than when the mirror plate 204 is in the process of beingdeflected. Therefore, in the “hold stage” after the addressing stage thevoltage applied to the selected electrodes 126 can be reduced from itsoriginal required level without substantially affecting the state ofdeflection of the mirror plates 204. One advantage of having a lowerhold stage voltage is that nearby undeflected mirror plates 204 aresubject to a smaller electrostatic attractive force, and they thereforeremain closer to a zero-deflected position. This improves the opticalcontrast ratio between the deflected mirror plates 204 and theundeflected mirror plates 204.

[0090] With the appropriate choice of dimensions (in one embodiment,spacer 210 separation between the mirror plate 204 and the electrode 126of 1 to 5 microns and hinge 206 thickness of 0.05 to 0.45 microns) andmaterials (such as single crystal silicon (100)), a reflective SLM 100can be made to have an operating voltage of only a few volts. Thetorsion modulus of the hinge 206 made of single crystal silicon may be,for example, 5×10¹⁰ Newton per meter-squared per radium. The voltage atwhich the electrode 126 operates to fully deflect the associated mirrorplate 204 can be made even lower by maintaining the mirror plate 204 atan appropriate voltage (a “negative bias”), rather than ground. Thisresults in a larger deflection angle for a given voltage applied to anelectrode 126. The maximum negative bias voltage is the releasingvoltage, so when the addressing voltage reduced to zero the mirror plate204 can snap back to the undeflected position

[0091] It is also possible to control the mirror plate 204 deflectionsin a more “analog” manner. Voltages less than the “snapping voltage” areapplied to deflect the mirror plate 204 and control the direction inwhich the incident light is reflected.

[0092] Alternate Applications:

[0093] Aside from video displays, the spatial light modulator 100 isalso useful in other applications. One such application is in masklessphotolithography, where the spatial light modulator 100 directs light todevelop deposited photoresist. This removes the need for a mask tocorrectly develop the photoresist in the desired pattern.

[0094] Although the invention has been particularly shown and describedwith reference to multiple embodiments, it will be understood by personsskilled in the relevant art that various changes in form and details canbe made therein without departing from the spirit and scope of theinvention. For example, the mirror plates 204 may be deflected throughmethods other than electrostatic attraction as well. The mirror plates204 may be deflected using magnetic, thermal, or piezo-electricactuation instead.

We claim:
 1. A micro mirror comprising: a mirror plate; a spacer supportframe; a hinge connected to the spacer support frame and the mirrorplate for allowing the mirror plate to rotate relative to the spacersupport frame about an axis defined by the hinge; and wherein each ofthe mirror plate, the spacer support frame, and the hinge are fabricatedfrom a single continuous piece of material.
 2. The micro mirror of claim1, wherein the material is single crystal silicon.
 3. The micro mirrorof claim 1, wherein the hinge is a vertically oriented torsion spring.4. The micro mirror of claim 1, wherein the mirror plate has areflective top surface.
 5. The micro mirror of claim 1, furthercomprising a reflective layer on top of the plate.
 6. The micro mirrorof claim 1, wherein the spacer support frame includes walls, the wallshaving a thickness of about 1 micron or less.
 7. The micro mirror ofclaim 1, further comprising a mechanical stop for stopping rotation ofthe mirror plate relative to the spacer frame when the mirror plate hasrotated to a predetermined angle.
 8. An array of a plurality of micromirrors, comprising: a spacer support frame with walls defining aplurality of cavities, each cavity corresponding to a micro mirror; aplurality of mirror plates; a plurality of hinges, each hinge connectedto at least one wall of the spacer support frame and connected to amirror plate of the plurality of mirror plates for allowing that mirrorplate to rotate relative to the spacer support frame about an axisdefined by the hinge; and wherein the spacer support frame, theplurality of mirror plates, and the plurality of hinges are fabricatedfrom a single continuous piece of material.
 9. The array of claim 8,wherein the mirror plates each have an upper surface.
 10. The array ofclaim 9, wherein the upper surfaces of the mirror plates are polished toreflect light.
 11. The array of claim 9, wherein a reflective layer isdeposited on each upper surface of the mirror plates for reflectinglight.
 12. The array of claim 8, further comprising a control substrateconnected to the spacer support frame and having at least one electrodecorresponding to each of the plurality of mirror plates for receiving avoltage to controllably deflect the mirror plate of the micro mirror.13. The array of claim 12, wherein the hinge divides the mirror plateinto a first part and a second part, such that when the first part ofthe mirror plate moves toward the control substrate as the mirror platerotates about the axis defined by the hinge, the second part of themirror plate moves away from the control substrate.
 14. The array ofclaim 12, wherein the control substrate further comprises addressing andcontrol circuitry for selectively applying voltages to the plurality ofelectrodes to selectively controllably deflect the mirror plates in thearray.
 15. The array of claim 8, wherein the surfaces of the plates inthe mirror array make up at least 85% of the surface area of the array.16. The array of claim 8, wherein the surfaces of the plates in themirror array make up at least 90% of the surface area of the array. 17.An array of a plurality of micro-mirrors, comprising: for each micromirror in the array, at least one mirror plate with an upper surface;for each micro mirror in the array, at least one hinge connected to theat least one mirror plate of that micro mirror for allowing that mirrorplate to rotate; a support frame with a plurality of support walls, eachhinge connected to at least one support wall, for supporting the hingeand the mirror plate and separating each mirror plate from a secondsubstrate connected to the support frame; and wherein the mirror platesand the hinges are fabricated from a single continuous piece ofmaterial.
 18. The array of claim 17, wherein there is a gap of 0.2microns or less between an edge of the upper surface of the mirror plateand a support wall of the support frame.
 19. The array of claim 17,wherein the upper surfaces of the mirror plates are substantiallyrectangular in shape.
 20. The array of claim 19, wherein the uppersurfaces of the mirror plates have an area of approximately 225 squaremicrons.